1) Technical Field of the Invention
The present invention relates to a semiconductor optical device, and in particular, relates to the semiconductor optical device including an active layer having a quantum well structure including a plurality of quantum well layers and barrier layers, in which each of the quantum well layers is doped with a dopant impurity producing p-type conductivity.
2) Description of Related Arts
In semiconductor optical device including the active layer having a quantum well structure, positive and negative electrodes (referred to simply as p- and n-electrodes, respectively) sandwich the active layer. A plurality of holes and electrons are injected from the p- and n-electrodes, respectively, and reach the active layer. In the active layer of the quantum well structure having a plurality of quantum well layers and barrier layers, holes and electrons meet and couple with each other within the quantum well layers, and thereby emitting light.
Since electrons, in general, have a mobility greater than that of holes, more electrons are injected into the quantum well layers than holes. Thus, an excess of electrons that contribute no light emission are left in the quantum well layers. Disadvantageously, the excessive electrons absorb the light emitted by the coupling of holes and electrons, thereby decreasing optical gain of the semiconductor optical device. Also, the excessive electrons reduce switching rate of the semiconductor optical device.
To solve those disadvantages, one approach has been proposed, in which the quantum well layer is doped with zinc as a hole source so as to quench the excessive electrons with the doped holes, thereby improving the optical gain of the semiconductor optical device. However, in practice, the zinc doping adversely causes the half-value width of the wavelength to be wider, and the threshold current and the operation current to be increased. The present inventors have studied the mechanism of the aforementioned problems and found that a substantial amount of zinc in the quantum well layers is thermally diffused into the well layers during the forming process of the semiconductor optical device.
JPA 06-334258 discloses a semiconductor laser device having a multiple quantum well structure including quantum well layers that are doped with zinc as a p-type impurity. However, it addresses prevention of the emission recombination current in the barrier layers by doping with the p-type impurity in quantum well layers with compression distortion. It fails to disclose a motivation to cancel the excessive electrons in the quantum well layers with the holes. In addition, JPA 06-334258 teaches that the p-type impurity of zinc would thermally be diffused so that many problems, such as the increased threshold current, occur, as described above.
Therefore, one aspect of the present invention has an object to provide a semiconductor optical device with an improved optical gain and an enhanced switching characteristics.
A first aspect of the present invention is to provide a semiconductor optical device including a positive and negative electrodes for providing holes and electrons, respectively. The semiconductor optical device also includes an active layer provided between the positive and negative electrodes. The active layer includes a multiple quantum well structure having a plurality of quantum well layers and barrier layers. The quantum well layers are doped with a p-type impurity less diffusible than zinc so that a plurality of trapping holes are produced and a plurality of excessive electrons contributing no light emission are quenched by the trapping holes. The p-type impurity can be beryllium, magnesium, or carbon.
A second aspect of the present invention is to provide a process for forming a semiconductor optical device includes growing a lower cladding layer on a semiconductor substrate at a first growth rate. Also, the process includes growing an active layer over the lower cladding layer, the active layer including a multiple quantum well structure having a plurality of quantum well layers doped with a p-type impurity less diffusible than zinc and a plurality of barrier layers. Further, the process includes growing an upper cladding layer over the active layer. The active layer is grown at a second growth rate that is about 1.5 to about 2.5 times of the first growth rate. The lower and upper cladding layers and the active layer are grown by MOCVD.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the sprit and scope of the invention will become apparent to those skilled in the art from this detailed description.